1. Field of the Invention
The present invention relates to an X-ray computer tomography apparatus which obtains a tomographic image of a subject by use of an X-ray.
2. Description of the Related Art
In a case where a contrast medium is utilized in an X-ray computer tomography apparatus, it is important to perform main scanning at a timing when the contrast medium administered to a subject flows into a portion (portion to be imaged) from which a tomographic image is to be obtained. The main scanning is scanning which collects data to reconstruct the tomographic image for diagnosis.
In Jpn. Pat. Appln. KOKAI Publication No. 2003-245275, a technology is disclosed in which preparatory scanning (prep scanning) is performed before the main scanning, an inflow situation of the contrast medium into the portion to be imaged is monitored based on a change of a CT value measured by this prep scanning, and the main scanning is automatically started in accordance with the monitored situation.
In general, the prep scanning is performed with respect to an object which is a portion different from the portion to be imaged. For example, in a case where the portion to be imaged is a liver, the prep scanning is performed with respect to an abdomen aorta as the object. Therefore, the main scanning is started with elapse of a certain time after the CT value measured by the prep scanning exceeds a threshold value, in order to wait until the contrast medium sufficiently flows into the portion to be imaged.
However, there is an individual difference in a time required for the contrast medium to reach the portion to be imaged from the portion which is the object of the prep scanning. Therefore, when the waiting time required until the main scanning is started is set to be constant as described above, there is a case where the main scanning is not started at an optimum timing.
Additionally, when axial scanning is performed along a rotation axis in addition to rotary scanning around the rotation axis during helical scanning or the like, a broad region is photographed in a direction along the rotation axis. To perform such tomography, even in a case where the main scanning is started at an appropriate timing, unless the axial scanning is performed at an appropriate speed during the main scanning, the portion to be imaged deviates from a bolus peak of the contrast medium, and the main scanning might not be appropriately performed.
In U.S. Pat. No. 6,337,992, a technology is disclosed in which a curve indicating a change of an X-ray decay value is estimated based on the change of the X-ray decay value immediately after starting injection of the contrast medium, and the helical scanning is started at a time when this curve is expected to come close to its maximum.
However, a step of estimating the above-described curve is very complicated, and it is difficult to achieve high precision. Therefore, if this curve has a large error, the timing to start the helical scanning might deviate from an optimum timing.